Over here in the UK, there's an adaptor available to interface a 2v pre-out head unit to the original Audi/Nokia/Gamma system. It actually needs a decent 2v+ pre-out signal to work properly, so should be ideal for the Empeg. If you can't track one down in your part of the world, we could probably ship one out to you for a reasonable enough price.

With the rest of the system, I'd generally agree with all that tanstaafl's said. Assuming you've only got fullrange drivers at the moment, then either a high-grade coaxial, or component kit in the original positions in the front of the car ought to brighten up the top end significantly. If you're looking for decent sound quality, as opposed to SPL, then running the fronts off a reasonably large high quality amp is probably a good idea - if so, then high pass filtering these at a fairly low fequency (probably 60-80hz) just to stop them distorting at high volume levels may or may not be necessary.

I think personally, if it was my own car, I'd be inclined to try and uprate the rear speakers to 6" as tanstaafl suggests, but run them bandpassed just to give mid-bass. You might be able to do this using the existing amps (if you can get an active crossover, or inline phono passive crossver into the signal line), as these are reasonably powerful, although a lot of them tend to be phsically attached to the speakers themselves - in which case, a reasonable budget amp is probably an easier option, and shouldn't degrade the overall sound quality, as it'll only be running a relatively narrow frequency band.

With the sub end of the system, I'd definitely be inclined to go for a 10" sub (or better still, a pair, if you're prepared to lose the necessary space, and can actually fit two in). The sub's really going to have to work for a living, as there's not much air passage into the car - that being the case, it's probably one of the rare occasions when I'd recommend a properly designed ported enclosure. Sealed boxes almost always sound better, but a ported box should give you a precious few more dB.

Dominic

Cambridge Car Audio
http://www.cambscaraudio.co.uk